With a tape measure, begin at the front of your hairline and wrap the tape behind your ear to the nape of your neck around to your other ear until the tape meets again at the front of your hairline. 95% of the population will fit into the average wig size. The comfortable elastic/cloth material stretches and fits head sizes from 21.5"-22.5". Cap sizes may vary slightly by brand. Most wigs have adjustable tabs at the neck to adjust the cap up to 1" for a comfortable and secure fit. Generally speaking if hats are always too small or always too big, you already know that you don’t have an average size head.